Overall: Tissue and organ-specific human B cell immunity Project Summary Experiments conducted using genetically modified mice and model pathogens or antigens have shaped our current understanding of adaptive immunity. From these studies, it is clear that both B and T cell immunity is exquisitely tailored to the pathogen or antigen. Furthermore, the data show that adaptive immune responses in lymphoid and peripheral tissues, which are found in both mucosal and systemic sites, can differ dramatically. Thus, we now know that studying immunity in a single lymphoid tissue to model antigens, while informative, does not reveal the full complexity of the adaptive immune response. This lesson, learned using mouse models, can also be applied to human immune responses. To date, the vast majority of human immune responses have only been queried in the blood. This approach has not only limited our analysis to the lymphocytes that are in circulation but has also restricted us to examining a small fraction of the cells (typically 1-2%) that are in circulation at a given time. This ?limited sampling approach? has also impaired our capacity to examine antigen- specific lymphocytes as these cells are rare populations, even during an ongoing immune response, and are largely undetectable under homeostatic conditions. Thus, we know exceedingly little about the phenotype, molecular programming, lifespan and function of human immune cells that reside primarily in tissues under homeostatic conditions. Moreover, we know essentially nothing about the clonal connections between antigen- specific cells in the different tissues of the human body. These represent fundamental gaps in our basic knowledge of the human immune system and reduce our ability to design new vaccines to prevent emerging infections, to create immunotherapies that can be used to treat cancer, autoimmunity and chronic disease and to develop tolerance-inducing regimens that will improve transplantation outcomes. Therefore, the overall goal of this U19 Program is to determine the molecular and functional relationships between human memory B lymphocyte and antibody secreting cell (ASC) populations that are found in pulmonary, intestinal and adipose tissues under homeostatic conditions. This U19 Program, which is composed of three Projects and three scientific Cores, will test our central hypothesis that antigen-specific, antigen-experienced human tissue-residing B cells are heterogeneous with respect to phenotype, transcriptome, breadth of reactivity, and function. We expect our studies to reveal the identity of novel subsets of antigen-experienced B cells exhibiting unique tissue- specific ?signatures?. This outcome will inform future studies of fundamental basic human immunobiology and is likely to influence future translational and clinical studies, as B cells specific for the viral antigens and bacterial cell wall components are important targets for vaccination and are required for host defense against a number of different Biodefense Category A and C pathogens.